1. Field of Invention
The present invention relates to an apparatus and method for forming a thermoplastic resin foam. The apparatus comprises a screw cylinder and a screw provided drivably in the directions of plasticization and injection inside said screw cylinder. The method comprises the step of injecting an inert gas such as a carbon dioxide gas or a nitrogen gas, having at least in pressure a pressure equal to or greater than a supercritical pressure into said screw cylinder. The method also comprises the steps of permeating melted resin with the inert gas under a supercritical state inside the screw cylinder, and injecting the melted resin, permeated by the inert gas, into a mold to provide a thermoplastic resin foam.
2. Related Art
There have been suggested many apparatuses and methods for molding a thermoplastic resin foam, for example, in Japanese Unexamined Patent Publication Hei 8-258096 and No. Hei. 10-230528. The methods include the steps of melting thermoplastic resin inside the cylinder of an injection-molding machine, permeating the melted thermoplastic resin with an inert gas such as carbon dioxide gas or nitrogen gas under a supercritical state, and injecting the melted resin, permeated by the gas, into the mold. The apparatus for forming a fine foam, disclosed in the aforementioned Japanese Unexamined Patent Publication Hei. 8-258096, is constructed in general as follows. That is, the apparatus comprises a heating cylinder, a main screw provided inside the heating cylinder, a mixing screw provided on the top end portion of the main screw, and an inert gas supply unit for supplying an inert gas to the mixing screw portion. Thus, when the main screw is rotatably driven to feed pelletized resin material toward the top end portion of the heating cylinder, the pelletized resin material is melted and then further uniformly melted with the mixing screw. At this time, when a carbon dioxide gas is supplied to the melted resin, the carbon dioxide gas permeates the melted resin material. Injecting the melted resin material permeated by the carbon dioxide gas into a mold by driving the main screw in the axial direction will provide a fine foam. On the other hand, the apparatus for forming a thermoplastic resin foam, disclosed in Japanese Unexamined Patent Publication Hei. 10-230528, is constructed as follows. That is, the apparatus comprises two separate units such as a successive plasticizing unit having a heating cylinder and a screw, and an injecting unit having a plunger. Thus, the two units can also provide a thermoplastic resin foam as follows. That is, when the screw is rotatably driven to melt the pelletized resin material and a carbon dioxide gas is supplied to the melted resin, the carbon dioxide gas permeates the melted resin material. The melted resin material permeated by the carbon dioxide gas is injected into the mold of the injecting unit having the plunger by driving the screw in the axial direction. Then, the plunger is driven to provide a thermoplastic resin foam in the same manner.
However, for example, the critical pressure of the carbon dioxide gas is 7.4 MPa. There is an unsolved problem of sealing the carbon dioxide gas inside the heating cylinder, which will arise when the carbon dioxide gas under a supercritical state with a pressure considerably higher than the aforementioned pressure is injected into the melted resin material inside the heating cylinder. Furthermore, the apparatus for forming a thermoplastic resin foam is complicated in structure and comparatively expensive.
The present invention was developed to provide a method for forming a thermoplastic resin foam, by which the aforementioned problems can be solved. More specifically, the object of the invention is to provide a method and an apparatus for forming a thermoplastic resin foam, by which foaming can be prevented inside the screw cylinder to provide a high-quality fine foam.
To achieve the aforementioned object, the present invention provides an apparatus for forming a thermoplastic resin foam, which is constructed as follows. That is, the apparatus comprises a screw cylinder having a material supply hole arranged near a rear end portion of one end of the screw cylinder, and an injection nozzle arranged on a top end portion of the other end of the screw cylinder. The apparatus also comprises a screw provided in said screw cylinder for being rotatably driven in directions of plasticization and injection, and a drive means for driving said screw in the directions of plasticization and injection. The apparatus is constructed such that said screw corresponds to said screw cylinder and is selected as a first metered, or plasticizing, portion, a low-pressure portion, and second metered, or plasticizing, portion in that order from the rear end portion to the top end portion. A gas supply hole for injecting an inert gas is disposed at a position corresponding to the low-pressure portion of said screw of said screw cylinder. The inert gas is a carbon dioxide gas or a nitrogen gas having, at least in pressure, a pressure equal to or greater than a supercritical pressure or under a supercritical state. Said drive means comprise an electric servomotor.
The present invention provides a method for forming a thermoplastic resin foam comprising the step of plasticizing a thermoplastic resin material by rotatably driving a screw provided drivably in directions of plasticization and injection inside a screw cylinder. The method also comprises the step of injecting melted resin, permeated by an inert gas, into a mold by driving said screw in the direction of injection after having injected the inert gas into said screw cylinder to allow the inert gas to permeate melted resin. Here, the inert gas is a carbon dioxide or a nitrogen gas having, at least in pressure, a pressure equal to or greater than a supercritical pressure or under a supercritical state. The method employs an electric servomotor as a drive means for driving said screw in the directions of plasticization and injection. After the step of plasticizing the thermoplastic resin material has been completed, said screw is also prevented from retreating by applying brake to said electric servomotor to thereby maintain a pressure at a supercritical pressure or more inside said screw cylinder.
The present invention provides a method for forming a thermoplastic resin foam comprising the step of plasticizing a thermoplastic resin material by rotatably driving a screw provided drivably in directions of plasticization and injection inside a screw cylinder. The method also comprises the step of injecting melted resin, permeated by an inert gas, into a mold by driving said screw in the direction of injection after having injected the inert gas into said screw cylinder to allow the inert gas to permeate melted resin. Here, the inert gas is a carbon dioxide or a nitrogen gas having, at least in pressure, a pressure equal to or greater than a supercritical pressure or under a supercritical state. The method employs an electric servomotor as a drive means for driving said screw in the directions of plasticization and injection. Furthermore, the step of plasticizing the thermoplastic resin material is performed until immediately before the step of injecting the melted resin is initiated.
The present invention provides a method for forming a thermoplastic resin foam comprising the step of plasticizing a thermoplastic resin material by rotatably driving a screw provided drivably in directions of plasticization and injection inside a screw cylinder. The method also comprises the step of injecting melted resin, permeated by an inert gas, into a mold by driving said screw in the direction of injection after having injected the inert gas into said screw cylinder to allow the inert gas to permeate melted resin. Here, the inert gas is a carbon dioxide or a nitrogen gas having, at least in pressure, a pressure equal to or greater than a supercritical pressure or under a supercritical state. The method employs an electric servomotor as a drive means for driving said screw in the directions of plasticization and injection. Furthermore, even after the step of plasticizing the thermoplastic resin material has been completed, said screw is driven at low speeds in the direction of plasticization until immediately before the step of injecting the melted resin is initiated.
The present invention provides a method for forming a thermoplastic resin foam comprising the step of plasticizing a thermoplastic resin material by rotatably driving a screw provided drivably in directions of plasticization and injection inside a screw cylinder. The method also comprises the step of injecting melted resin, permeated by an inert gas, into a mold by driving said screw in the direction of injection after having injected the inert gas into said screw cylinder to allow the inert gas to permeate melted resin. Here, the inert gas is a carbon dioxide or a nitrogen gas having, at least in pressure, a pressure equal to or greater than a supercritical pressure or under a supercritical state. The method employs an electric servomotor as a drive means for driving said screw in the directions of plasticization and injection. Furthermore, when a pressure has dropped below a pre-set value inside said screw cylinder, said screw is driven in the direction of plasticization so as to stop said screw when the pressure becomes equal to or greater than the pre-set value. This is carried out to maintain the pressure at a supercritical pressure or more inside said screw cylinder.
The present invention provides a method for forming a thermoplastic resin foam comprising the step of plasticizing a thermoplastic resin material by rotatably driving a screw provided drivably in directions of plasticization and injection inside a screw cylinder. The method also comprises the step of injecting melted resin, permeated by an inert gas, into a mold by driving said screw in the direction of injection after having injected the inert gas into said screw cylinder to allow the inert gas to permeate melted resin. Here, the inert gas is a carbon dioxide or a nitrogen gas having, at least in pressure, a pressure equal to or greater than a supercritical pressure or under a supercritical state. The method employs an electric servomotor as a drive means for driving said screw in the directions of plasticization and injection. Furthermore, when said screw is driven in the direction of plasticization, driving said screw in the opposite direction of plasticization is intermittently combined therewith.
The present invention provides a method for forming a thermoplastic resin foam comprising the step of plasticizing a thermoplastic resin material by rotatably driving a screw provided drivably in directions of plasticization and injection inside a screw cylinder. The method also comprises the step of injecting melted resin, permeated by an inert gas, into a mold by driving said screw in the direction of injection after having injected the inert gas into said screw cylinder to allow the inert gas to permeate melted resin. Here, the inert gas is a carbon dioxide or a nitrogen gas having, at least in pressure, a pressure equal to or greater than a supercritical pressure or under a supercritical state. The method employs an electric servomotor as a drive means for driving said screw in the directions of plasticization and injection. Furthermore, during plasticization, said screw cylinder is provided with micro-vibration in the direction of injection.
To achieve the aforementioned object, the present invention provides an apparatus for forming a thermoplastic resin foam, which is constructed as follows. That is, the apparatus comprises a screw cylinder having a material supply hole arranged near a rear end portion of one end of the screw cylinder, and an injection nozzle arranged on a top end portion of the other end of the screw cylinder. The apparatus also comprises a screw provided in said screw cylinder for being rotatably driven in directions of plasticization and injection, and a drive means for driving said screw in the directions of plasticization and injection. The apparatus is constructed such that said screw corresponds to said screw cylinder and is selected as a first metered, or plasticizing, portion, a low-pressure portion, and second metered, or plasticizing, portion in that order from the rear end portion to the top end portion. A gas supply hole for injecting an inert gas is disposed at a position corresponding to the low-pressure portion of said screw of said screw cylinder. The inert gas is a carbon dioxide gas or a nitrogen gas having, at least in pressure, a pressure equal to or greater than a supercritical pressure or under a supercritical state. Said drive means comprise an electric servomotor.